1. Field of the Invention
The present invention relates to a liquid crystal display device and, more particularly, to an in-plane switching mode liquid crystal display device.
2. Description of the Related Art
Liquid crystal display devices are typically used because they consume low power and provide high picture quality. A liquid crystal display device is formed by attaching face to face a thin film transistor array substrate and a color filter substrate with a uniform interval therebetween, and disposing a liquid crystal layer between the thin film transistor array substrate and the color filter substrate.
Pixels are arranged on the thin film transistor array substrate in a matrix arrangement. A thin film transistor, a pixel electrode and a capacitor are formed within a pixel. A common electrode, an RGB color filter and a black matrix are formed on the color filter substrate. The common electrode applies an electric field to the liquid crystal layer together with the pixel electrode. The RGB color filter provides color display capabilities. An alignment film is formed at facing surfaces of the thin film transistor array substrate and the color filter substrate and is rubbed to orient the liquid crystal layer in a specified direction.
When an electric field is applied between the pixel electrode and the common electrodes, the liquid crystal rotates due to a dielectric anisotropy. As a result, light is transmitted or blocked by pixels to display a character or an image. However, such a twisted nematic mode liquid crystal display device has a narrow viewing angle. In-plane switching mode LCD arrangements have been recently introduced to improve the narrow viewing angle by aligning liquid crystal molecules in an almost horizontal direction with respect to the substrate.
FIG. 1A depicts a plan view of an in-plane switching mode liquid crystal display (LCD) device in accordance with a related art arrangement. FIG. 1B illustrates a sectional view of an in-plane switching mode liquid crystal display (LCD) device in accordance with a related art arrangement. As shown in FIG. 1A, gate lines 1 and data lines 3 are arranged horizontally and vertically on a first transparent substrate 10, defining pixel regions. Although in an actual liquid crystal display device, the ‘N’ number of gate lines I and the ‘M’ number of data lines 3 cross each other to create an N×M number of pixels. Only one pixel is shown in FIG. 1A for explanatory purposes.
A thin film transistor 9 is disposed at a crossing of the gate line 1 and the data line 3. The thin film transistor 9 includes a gate electrode 1a, a semiconductor layer 5 and source/drain electrodes 2a and 2b. The gate electrode 1a is connected to the gate line 1. The source/drain electrodes 2a and 2b are connected to the data line 3. A gate insulation layer 8 is formed on the entire substrate.
A common line 4 is arranged parallel to the gate line 1 in the pixel region. A pair of electrodes, which are a common electrode 6 and a pixel electrode 7, are arranged parallel to the data line 3 for switching liquid crystal molecules. The common electrode 6 is simultaneously formed with the gate line 1 and connected to the common line 4. The pixel electrode 7 is simultaneously formed with the source/drain electrodes 2a and 2b and connected to the drain electrode 2b of the thin film transistor 9. A passivation film 11 is formed on the entire surface of the substrate 10 including the source/drain electrodes 2a and 2b. A pixel electrode line 14 is formed to overlap the common line 4 and is connected to the pixel electrode 7. The pixel electrode line 14, the common line 4, and the gate insulation layer 8 interposed therebetween, form a storage capacitor (Cst).
A black matrix 21 and a color filter 23 are formed on a second substrate 20, on which an overcoat film is formed for flattening the color filter 23. The black matrix 21 prevents light leakage to the thin film transistor 9, the gate line 1 and the data line 3. The color filter 23 provides color display capabilities to the liquid crystal display device. Alignment films 12a and 12b are formed at facing surfaces of the first and second substrates 10 and 20. The alignment films 12a and 12b determine an initial alignment direction of the liquid crystal. A liquid crystal layer 13 is formed between the first and second substrates 10 and 20. The light transmittance of the liquid crystal layer 13 is controlled by a voltage applied between the common electrode 6 and the pixel electrode 7.
FIG. 2A illustrates the orientation of a liquid crystal molecule in accordance with the related art in-plane switching mode LCD device when no voltage is applied to the LCD device. Referring to FIG. 2A, when no voltage is applied between the common electrode 6 and the pixel electrode 7 of the in-plane switching mode LCD device, a liquid crystal molecule in the liquid crystal layer is arranged along a rubbing direction (the direction indicated by arrow ↑ in the drawing) of the alignment film formed at the facing surfaces of the first and second substrates.
FIG. 2B illustrates the orientation of a liquid crystal molecule in accordance with the related art in-plane switching mode LCD device when a voltage is applied to the LCD device. Referring to FIG. 2B, when a voltage is applied between the common electrode 6 and the pixel electrode 7, an electric field is generated between electrodes 6 and 7, and the liquid crystal molecule transmits light according to the generated electric field.
FIG. 3 is a graph showing variations of the light transmittance characteristics of the related art in-plane switching mode LCD device. As shown in FIG. 3, the light transmittance increases linearly with the voltage applied between the common electrode 6 and the pixel electrode 7. However, if the voltage increases continuously beyond a maximum value, the light transmittance starts to decrease in a parabolic shape. In this case, the voltage Vmax corresponding to a maximum transmittance is obtained when the liquid crystal molecule makes a 45° angle with respect to the initial alignment direction of the alignment film. Moreover, the transmittance of the liquid crystal material is reduced if a voltage higher than Vmax is applied between the common electrode 6 and the pixel electrode 7.
However, the graph in FIG. 3 only depicts a theoretical transmittance. In an actual related art LCD device, the maximum luminance is reached at a voltage lower than the theoretical value Vmax. Thus, luminance is degraded by the application of the theoretical value Vmax to an actual product. Thus, in an actual related art LCD device, the maximum value of the applied voltage is set lower than the theoretical Vmax. Accordingly, maximum luminance of the product cannot generally be reached.
The in-plane switching mode LCD device suffers from the following problems. Liquid crystal molecules in the liquid crystal layer 13 are always oriented on the same plane, thus reducing a grey level in the vertical and horizontal viewing angle direction. Although the viewing angle can be enhanced, the transmittance at a voltage higher than Vmax is degraded. In addition, although a voltage Vmax needs be applied to achieve the brightest possible image, picture quality is impacted by the collective arrangement of liquid crystal molecules in one direction. For example, a yellow shift appears when a screen image is viewed in the direction of the shorter side of the liquid crystal molecules. A blue shift occurs when the screen image is viewed in the direction of the longer side of the liquid crystal molecules.